It's fitting that one of the the tallest timber frame buildings in the country should be built alongside the longest canal in Great Britain.

On a cold, but sunny day, light reflects off the Grand Union Canal, providing a natural spotlight on what might outwardly look a simple structure, but in fact hides a complex feat of timber engineering.

Residents of Grand Union Heights, in Glacier Way, Ealing, will be blissfully unaware that their building is a piece of history in the making – five to seven storeys of timber frame that push the boundaries of timber technology and deliver a cost-effective, functional – and attractive – solution to what was a complicated brief.

The mixed-tenure development comprises 108 one-, two- and three-bedroom flats for social housing, shared ownership and private residential. The site previously belonged to supermarket giant Sainsbury. When it realised its car park was under-used, it decided to maximise the development potential of the space and 'install' a large residential building.

Architects Hunter & Partners, working for main contractor United House Ltd, were given a tight brief with a number of conditions, not least that some of the car park be retained for Sainsbury's customers' use.

This meant creating a concrete podium slab at first floor level with large down-stand beams, 1.2m deep, and pillars to support the timber frame structure above. “If we had gone for a traditional structure, the loading would have been astronomical,” said Mark Baines, director at Hunter & Partners. “The down-stand beams alone would have been 2m thick which á Ü would have provided a headroom problem in the car park.”

A light gauge steel frame was considered, but the difficulties in using this above three or four storeys, and the costs involved in getting it to work, would have negated its use. Not only that, but rocketing steel prices at the time meant that timber frame was the obvious choice.

“The contractor said 'let's push the limit',” said Baines. Hunter & Partners had already worked with subcontractor Stewart Milne Timber Systems (SMTS) on another project in west London and, with this experience in mind, decided to see whether timber frame would again be viable for a high-rise development of this type.

There were several preplanning meetings with SMTS to make sure the scheme would work as timber frame. “Unusually, in this case we were dealing with our contractor's subcontractor – before the planning application had even gone to the local authority,” said Baines.

The height of the building presented other challenges. The BRE had tested seven-storey accommodation in timber frame but not the means of escape. This meant coming up with a workable solution, particularly given that the vertical circulation cores in all the buildings were built of steel, providing standard concrete and steel escape stairs (pp62/63). But, because these cores were surrounded by timber frame, Hunter & Partners had to allow for differential settlement at threshold level in all of the cores, as well as the slip joints used to anchor the otherwise free-standing balconies to the building.

“This made it desperately complicated,” said Baines. It was also the first timber frame building the main contractors had attempted. Cutting teeth on a seven-storey building was certainly a brave choice, and United House did need some educating in terms of the construction logistics.

The site is adjacent to – and was previously owned by – Sainsbury

“As a design and build company, it was used to following a certain order in the building programme,” explained Baines. However, the nature of timber frame – particularly one of seven storeys – meant the architects needed to account for 7mm of shrinkage for every floor.

“A lot of this shrinkage will come out with the sheer weight of all the building components, which meant getting all the plasterboard into the structure, whether it was on the walls or sitting on pallets waiting to be installed,” said Baines.

Extra weight also came in the form of fire protection – 90 minutes for a seven-storey building – which meant using one layer of Gyproc 12.5mm Soundbloc board over one layer of 19mm Gyproc Fireline Duplex around the timber frame to make it fireproof.

“The amount of weight going into this building was enormous,” said Baines. “It all had to be loaded up and the roof put on before the cladding was fixed into place, otherwise you would have had more settlement to take up in the movement of the joints outside the building.”

With the Colindale fire making national headlines (Timber Building Winter 2006), the fire protection capabilities of multi-storey timber frame have come under scrutiny.

“We took a lot of calls about what effect Colindale would have on our plans to push forward in timber frame,” said Baines. “But it shouldn't make any difference; you still work to the same Building Regulations and still have the same integrity as you would with a traditional build structure.”

Where he does believe timber frame differs is during construction: there should be stringent checks when operations like welding take place because the risks are that much greater. “But if there's proper site management and safety, it shouldn't be any more risky than a traditional building,” he added.

That aside, the biggest challenge was differential settlement. “If everything is made in timber frame, it shrinks and settles at the same rate,” explained Baines. “It's when you mix structural methodologies you get problems.”

The building's exterior was originally designed as wet render but it was felt that this couldn't take the shrinkage in the external fabric and a panellised system of cladding was used instead, including western red cedar from LSC.

Due to the restricted and complex nature of the site – and to reduce the overall disruption to Sainsbury‘s – the timber frame system was fabricated by SMTS off-site and brought to the development as finished floor and wall cassettes.

It was certainly quick to erect. SMTS was putting up a storey a week and the whole construction process, from podium slab to finished building, took around 18 months – providing the contractor a saving of four months off the original build programme, as well as significant cost savings. In the end, the entire development was bought by affordable housing provider Network Housing Group, including the private sale.

Timber frame also beat other materials in terms on environmental credentials. “Concrete is one of the most carbon heavy production techniques,” said Baines. “Anything you can do to minimise its use has to be the way forward.”

All the timber frame used in the building was FSC-certified, he added, ticking many of the right boxes under the Ecohomes specifications.

With its performance, cost savings and 'green' benefits, there's no doubt that timber frame will see increasing use in bigger, and taller, buildings: Grand Union Heights, in Glacier Way, is just the tip of the iceberg.

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